This invention relates to a dosimeter for measuring the dose of a charged particle beam, and more particularly to a charged particle beam-dosimeter which has a wide dynamic range without changing-over the ranges.
FIG. 3 is a circuit diagram, partly in blocks, showing a charged particle beam-dosimeter in the prior art. As shown in the figure, the charged particle beam-dosimeter in the prior art comprises an ionization chamber 2 which produces an ionization current having a magnitude proportional to the dose of a charged particle beam 1, an operational amplifier 3 the inverting input terminal of which is connected to the ionization chamber 2 and the noninverting input terminal of which is grounded, a group of resistors 4 which are connected in parallel with the operational amplifier 3, specifically between the inverting input terminal and the output terminal thereof, a group of relays 5 which serve to switch the group of resistors 4, a transfer switch 6 which serves to select the group of relays 5, a voltage-to-frequency converter 7 which is connected to the output terminal of the operational amplifier 3, and a counter 8 which is connected to the output side of the voltage-to-frequency converter 7. Further, the group of resistors 4 consist of resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4 which are respectively connected in series with a plurality of, for example, four relay contacts RY.sub.1a, RY.sub.2a, RY.sub.3a and RY.sub.4a, and the individual series circuits are connected in parallel with one another. The group of relays 5 consist of four relays in total, RY.sub.1, RY.sub.2, RY.sub.3 and RY.sub.4 which correspond respectively to the aforementioned relay contacts RY.sub.1a, RY.sub.2a, RY.sub.3a and RY.sub.4a, and the individual relays are connected in parallel with illustrated diodes between a power source + and the respective stationary contacts S.sub.1, S.sub.2, S.sub.3 and S.sub.4 of the transfer switch 6.
With the charged particle beam-dosimeter of the prior art constructed as explained above, when the movable contact piece M of the transfer switch 6 is moved from its illustrated state so as to come into touch with the stationary contact S.sub.2, the relay RY.sub.2 is energized and its relay contact RY.sub.2a is closed by current which flows from the power source + to ground through the relay RY.sub.2, stationary contact S.sub.2 and movable contact piece M. Consequently, the resistor R.sub.2 is electrically connected in parallel with the operational amplifier 3. When, under this state, the ionization chamber 2 feeds the node between the inverting input terminal of the operational amplifier 3 and the group of resistors 4 with the ionization current I.sub.s whose magnitude is proportional to the dose of the charged particle beam 1, the fed current I.sub.s is consumed by a voltage drop across the resistor R.sub.2, and the operational amplifier 3 balances at an output voltage at which the node becomes 0 V. Accordingly, the measurement current range of the dosimeter is determined by the resistance of the resistor R.sub.2 and the supply voltage of the operational amplifier 3. The above output voltage is expressed by R.sub.2 .multidot.I.sub.s, and the sign thereof becomes minus in the direction in which the current I.sub.s flows from the ionization chamber 2 to the node, whereas it becomes plus in the direction in which the current flows from the node into the ionization chamber 2. When the resistance of the resistor R.sub.2 is set at 10 k.OMEGA., the sensitivity of the dosimeter is 100 .mu.A/V. In a case where the range is to be changed-over, the movable contact piece M is brought into touch with the stationary contact S.sub.1 or S.sub.3, and the relay RY.sub.1 or RY.sub.3 is energized to close its relay contact RY.sub.1a or RY.sub.3a. Thus, when the resistor R.sub.1 of 1 k.OMEGA. and the resistor R.sub.3 of 100 k.OMEGA. are used by way of example, the sensitivities become 1 mA/V and 10 .mu.A/V, respectively. However, the maximum sensitivity of the dosimeter is restricted by the input bias current of the operational amplifier 3, while the minimum sensitivity is restricted by the lowest load resistance thereof. The output voltage of the operational amplifier 3 is converted into a frequency by the voltage-to-frequency converter 7, and the frequency is counted by the counter 8.
As stated above, the charged particle beam-dosimeter of the prior art has had such a problem that the ranges must be changed-over on each occasion in order to change the sensitivity.